PSI - Issue 2_B

Chretien Gaëlle et al. / Procedia Structural Integrity 2 (2016) 950–957

951

2

Gaëlle Chretien et al. / Structural Integrity Procedia 00 (2016) 000–000

Since the initial work of Pearson (1975), many investigations using continuum approaches have highlighted that growth rates of small cracks can be significantly greater than the corresponding rates of long cracks when characterized in terms of the same nominal driving force. Indeed, anomalous short crack growth behavior has been observed on several Titanium alloy: IMI685 (Brown and Hicks (1983)), Ti-6Al-4V (Sinha et al. (2000), Berata (1992), Oberwinkler et al. (2011), Tokaji (2006)), Ti6246 (James and Morris (1988)). Moreover short cracks have been shown to propagate below the threshold for the propagation of long cracks and to higher growth rates, this was related to the lack of crack closure (Zeghloul and Petit (1985), Pineau (1986), Ritchie and Yu (1986), McClung and Sehitoglu (1988), Ravichandran and Ritchie (1999), Petit (1984)), in opposition to the high level of crack closure commonly observed at near-threshold for long cracks. The most famous criteria of non-propagation is Kitagawa and Takahashi (1976) diagram which represents the evolution of propagation threshold in function of the crack length a. This criterion allows to qualitatively understand short crack behavior and to design for infinite life. According to Suresh and Ritchie (1984), different types of short cracks dependent on their length can be distinguished on this diagram: (i) Microstructurally small cracks, whose size is comparable to the scale of the characteristic microstructural dimension; (ii) Mechanically small cracks, for which size is comparable to the near-tip plasticity; (iii) Physically short cracks, which are significantly larger than the characteristic microstructural dimension and the scale of the local plasticity. The present study focuses on the characterization of the near-threshold propagation of 2D physically through- thickness short cracks with an initial depth larger than 5 times the average grain size and only short in one dimension (crack depth). The length of these fatigue cracks, typically of the order of 0.1–2.0 mm in length, is significantly longer than both the scale of the microstructure (grains size of 15 µm) and the size of the near-tip yielding zone. Such cracks are relevant to the third type of this small crack classification and confirm the applicability of the linear elastic fracture mechanic (LEFM) concepts. Since crack closure mechanisms arise as a result of premature contact between the crack faces, and since, by definition, a short crack has a limited wake, crack closure effects are generally less pronounced than for a long crack. The experimental difficulties to set up experiments of crack propagation at elevated temperatures with crack opening measurements imply that studies on the fatigue behavior at elevated temperatures in Ti-6Al-4V alloy have been a limited number (Berata (1992)). Especially short crack growth at elevated temperatures in high cycle fatigue regime has not been studied. The present study has been undertaken to investigate the influence of the crack length on the threshold for crack arrest in a Titanium alloy. Physically through-thickness short cracks artificially obtained in CT specimens are tested in ambient air at 20°C and 400°C. The experimental results are then investigated in order to establish the Kitagawa-Takahashi diagrams of this material in these conditions of solicitation. The role of crack closure is particularly taken into account.

Nomenclature a

Crack length

CT

Compact Tension specimen Stress intensity factor Number of fatigue cycles Stress intensity factor Crack edge displacement Differential displacement Crack length of the short crack Stress intensity factor range Applied load

K N P

SIF

δ

δ’ Δa ΔK Δσ Δσ 0

Stress range

Stress range at fatigue limit

Effective Nominal Long crack Opening Threshold

eff

nom

LC

op th

2. Specimen and experimental set-up 2.1. Material and specimen

The material used is a Ti-6Al-4V alloy. The microstructure revealed by etching process using a Kroll’s reagent (2 mml HF + 5 ml HNO3 + 100 ml water), is a bi-modal microstructure consisting of equiaxed primary α p grains of about 15 µm in a lamellar matrix α +β as shown in figure 1. Fatigue crack growth tests are performed on CT40 specimens in accordance with ASTM E647 00 recommendation (Figure 2).